In recent years, from a viewpoint of global environmental protection, cases of introducing heat pump type air-conditioning apparatuses, using the air as a heat source, are increasing even in cold areas, in place of boiler type heating devices in which heating is performed by burning fossil fuel. In a heat pump type air-conditioning apparatus, heating can be performed more efficiently by the amount of heat supplied from the air in addition to the electricity input to the compressor.
On the other hand, however, in a heat pump type air-conditioning apparatus, when the temperature of the air outside the room (outside air) (outside air temperature) is decreasing, an outdoor heat exchanger, functioning as an evaporator to allow heat exchange between the outside air and refrigerant, is more likely to be frosted. Accordingly, it is necessary to perform defrosting to melt the frost deposited on the outdoor heat exchanger. As a method of performing defrosting, there is a method of reversing the flow of refrigerant in heating to supply the refrigerant from the compressor to the outdoor heat exchanger, for example. However, in this method, as defrosting is performed while stopping heating in the room in some cases, there is a problem that comfortability is impaired.
As such, to allow heating even during defrosting, a method has been proposed in which an outdoor heat exchanger is divided for example, and when a part of the outdoor heat exchanger performs defrosting, the other part of the outdoor heat exchanger functions as an evaporator to remove heat from the outside air to perform heating (see Patent Literature 1, Patent Literature 2, and Patent Literature 3, for example).
For example, in the technique described in Patent Literature 1, an outdoor heat exchanger is divided into two heat exchanger units. Then, in the case of defrosting one heat exchanger unit, an electronic expansion valve provided upstream of the heat exchange unit to be defrosted is closed. Further, by opening a solenoid valve of a bypass pipe for allowing refrigerant to bypass from a discharge pipe of the compressor to the inlet of the heat exchanger unit, a part of the high-temperature refrigerant discharged from the compressor is allowed to directly flow into the heat exchanger unit to be defrosted. Then, upon completion of defrosting of one heat exchanger unit, defrosting is performed on the other heat exchanger unit. At this time, in the heat exchanger unit to be defrosted, defrosting is performed in a state where the refrigerant therein is in a low-pressure state equivalent to the suction pressure of the compressor (low-pressure defrosting).
Further, in the technique described in Patent Literature 2, a plurality of heat source units and at least one indoor unit are provided. Then, in only a heat source unit provided with a heat source side heat exchanger to be defrosted, the connecting state of a four-way valve is reversed from the state at the time of heating, and the refrigerant discharged from the compressor is allowed to directly flow into the heat exchanger on the heat source unit side. At this time, in the heat exchanger on the heat source unit side to be defrosted, defrosting is performed in a state where the refrigerant therein is in a high-pressure state equivalent to the discharge pressure of the compressor (high-pressure defrosting).
Further, in the technique described in Patent Literature 3, an outdoor heat exchanger is divided into a plurality of outdoor heat exchangers, and a part of the high-temperature refrigerant discharged from the compressor is allowed to flow into the respective outdoor heat exchangers by turns, and defrosting is performed on the respective outdoor heat exchangers by turns. As such, heating can be performed continuously in the apparatus as a whole. Further, the compressor includes an injection port, and the refrigerant supplied to the outdoor heat exchanger to be defrosted is injected from the injection port into the compressor. At this time, in the outdoor heat exchanger to be defrosted, defrosting is performed in a state where the pressure of the refrigerant therein is lower than the discharge pressure of the compressor and higher than the suction pressure (pressure that becomes a temperature slightly higher than 0 degrees C. on a saturation temperature conversion basis) (medium-pressure defrosting). Among the three types of defrosting methods, Patent Literature 3 describes that defrosting can be performed more efficiently by medium-pressure defrosting, compared with the other methods.
Further, in the techniques described in Patent Literature 1 and Patent Literature 3, defrosting is terminated after it is performed for a certain period of time. Further, defrosting is terminated when the temperature of a temperature sensor, provided on the refrigerant outflow side of the heat exchanger to be defrosted, exceeds a predetermined temperature. In the technique described in Patent Literature 2, an expansion device controls the degree of subcooling (subcooling) on the refrigerant outflow side of the heat source side heat exchanger to be defrosted. It is configured that defrosting is terminated when it is determined that the opening degree of the expansion device becomes a predetermined opening degree or less.